Solid propellant rocket motor cases for missile systems, spacecraft boosters and other types of large and small high performance, lightweight pressure vessels are commonly made from fiber reinforcement and various formulations of polyepoxide resins (epoxy resins) by a filament winding process. Similarly, filament winding with both polyesters and epoxy resins has made possible production of lightweight tanks, poles, piping and the like. Historically, fiberglass has been the most common reinforcement fiber, but other fibers such as carbon filaments, boron filaments, and high modulus organic polymer filaments, most significantly aramid filaments, have become increasingly useful in these composite structures to take advantage of their differing and sometimes unique physical properties.
The resins utilized are typically epoxy formulations based on diglycidyl ether-bisphenol A (DGEBA), reactive low molecular weight epoxy diluents and curing agents such as aliphatic and aromatic amines and carboxylic acid anhydrides. Both flexibilized and rigid epoxy resins have been used as matrix resins for filament wound composite structures.
In providing composite articles, such as the aforesaid pressure vessels, one has employed either a wet winding process or a prepreg process. When the resin-fiber combination is to be employed in wet winding, the fiber is simply run through a resin bath containing the resin composition whereby the fiber is coated with the composition. The resulting resin-fiber combination is then wound directly into the desired structure. On the other hand, if a prepreg is to be used, the fiber or "tow" is impregnated with a curable resin composition and then the resulting "tow" wound on a spool as a prepreg and stored for winding at a future time. When the prepreg is converted into a composite article, the prepreg is then typically cured by polymerization by means of heat or radiation.
One drawback encountered in the production of composite pressure vessels has been the fall-off or reduction in pressure vessel tensile strength compared to the unidirectional, axial impregnated tow tensile strength. A common measure of performance in composite pressure vessels is fiber strength translation of such tow strength to delivered tensile strength of the hoop fibers of the composite pressure vessel. Improved fiber strength translation of even a few percent is significant and valuable since fiber strength translation directly effects the design, weight, strength and cost of such pressure vessels. Thus, a highly desirable object would be to increase the tow or fiber strength translation into delivered tensile strength of hoop fibers of composite pressure vessels expressed as a percent of the tow strength.
A further drawback resides in the variation of the material from which the composite pressure vessels are produced. For pressure vessels the material strength used in designing (or design allowable strength) is the average strength of the test pressure vessels less three times the coefficient of variation (CV). For example, if the average strength is 90% of the tow strength and the standard deviation is 3%, the design allowable strength is 82% of the tow strength i.e. ##EQU1## Historical precedent suggests that composite pressure vessels fabricated by wet-winding have CV's of about 4 to 8% while pressure vessels fabricated from prepregs have CV's of approximately 2-4%. It would therefore be highly desirable to significantly reduce the CV's to below these values.